Leviticus 18:14 - Matthew Poole's English Annotations on the Holy Bible

Of thy father's brother, i.e. of his wife, as the next words explain it. And as a man may not marry his aunt, so neither may a woman marry her uncle, there being altogether the same distance in kindred, and the selfsame reason of the law. And for the examples of Abraham, Amram, Othniel, &c., to the contrary, they were before the publication of this law, by which it pleased God to restrain the liberty allowed formerly, when the holy seed was in a narrower compass, and fewer persons, which altered the case. For in that regard there was a time when God allowed brethren and sisters to marry, to wit, when there were no other in the world, which was the case of Adam's immediate children. We learn from hence that the same degrees are forbidden in consanguinity or kindred by blood, and in affinity or kindred by marriage. She is thine aunt: some infer from hence that it is unlawful for cousin-germans, or the children of brethren and sisters, to marry. But there is not the same reason, nor the same degree of distance, for my uncle or aunt are nearer akin to me than their children are. Yet because it seems doubtful to many, and may hereafter prove occasion of grievous perplexities of mind, especially to tender and scrupulous consciences, Christian prudence directs us to choose the safest way, there being so great a latitude of unquestionable persons.